Analysis of forensic polymer samples using double shot pyrolysis gas chromatography – Mass spectrometry

Abstract

Polymers are present in many different products, such as paints, plastics, and rubbers, which are routinely encountered in forensic casework. Comparison of such samples involves an initial visual examination followed by comparison of the chemical compositions of the exhibits. Techniques such as Fourier transform infrared spectroscopy (FTIR) and pyrolysis gas chromatography – mass spectrometry (PyGC-MS) have been reported for determining the chemical compositions of polymers in forensic samples. Double-shot pyrolysis gas chromatography – mass spectrometry (DS-PyGC-MS) is an extension of single-shot pyrolysis gas chromatography – mass spectrometry (SS-PyGC-MS) which is the current PyGC-MS method used in most forensic laboratories. DS-PyGC-MS involves a preliminary thermal desorption GC-MS step, followed by the pyrolysis GC-MS step, with this second step being analogous to SS-PyGC-MS. The pyrolyser furnace operates at a lower temperature during the thermal desorption step, allowing low volatility compounds, such as additives, to be thermally desorbed and detected, minimising interference from the polymeric component of the sample. This pilot study analysed four different polymeric substrates, commonly encountered in forensic casework, by DS-PyGC-MS. The substrates chosen were tyre rubber, road cones, cling film, and shotgun wads. The aim was to investigate whether more chemical information was generated by DS-PyGC-MS compared to SS-PyGC-MS, potentially providing increased discrimination of such samples. Qualitative results showed that tyre rubber and road cones were ideal substrates for DS-PyGC-MS. A wide range of additives were detected in these samples in the thermal desorption step, which were not detected using SS-PyGC-MS. All of the rubber tyres (n=5) and road cones (n=6) were able to be uniquely distinguished using DS-PyGC-MS. Some additional compounds were detected in the thermal desorption analysis of shotgun wads (n=4), providing increased discrimination compared to SS-PyGC-MS. For the cling film samples analysed (n=7) the polyethylene-based cling films (n=6) could not be distinguished from each other, with no compounds detected in the thermal desorption step. The other cling film sample contained a mixture of polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT) and could easily be distinguished from the polyethylene-based cling films using either SS- or DS-PyGC-MS, or other common analytical methods such as Fourier transform infrared spectroscopy (FTIR). This pilot study has demonstrated that DS-PyGC-MS has the potential to provide more comprehensive chemical composition information for some polymeric substrates and is a promising method for the forensic comparison of polymer evidence.